This invention pertains generally to metallic alloys in amorphous form useful in the construction of electrical and magnetic devices, and more particularly to methods for optimizing the magnetic properties of amorphous metals when formed into configurations such as cores for electrical transformers.
Amorphous metals are principally characterized by a virtual absence of a periodic repeating structure on the atomic level, i.e., the crystal lattice, which is a hallmark of their crystalline metallic counterparts. The non-crystalline amorphous structure is produced by rapidly cooling a molten alloy of appropriate composition such as those described by Chen et al., in U.S. Pat. No. 3,856,513, herein incorporated by reference.
In one such process for producing amorphous metal in commercially practicable lots, the chill-block melt spinning process, a stream of molten metal of appropriate composition is ejected from a crucible onto one or more rapidly moving chill or substrate surfaces and rapidly quenched, i.e., cooling rates on the order of about 10.sup.4 to 10.sup.6 .degree. C./sec, to form an elongated thin ribbon-like body whose length is appreciably greater than either its width or thickness. Due to the rapid cooling rates, the alloy does not form in the crystalline state, but assumes a metastable non-crystalline structure representative of the liquid phase from which it was formed. Due to the absence of crystalline atomic structure, amorphous alloys are frequently referred to as "glassy" alloys. The quench rates achieved during the rapid quenching and solidification are controlled primarily by the thickness of the ribbon and the effectiveness of the interfacial contact between the substrate and the ribbon.
Depending upon the composition, many unique properties can be obtained in amorphous metals which cannot necessarily be obtained in crystalline metallic alloys. The unique magnetic properties of the amorphous alloys, for example, have attracted considerable attention. As a class, amorphous alloys exhibit higher magnetic permeabilities, i.e., the ratio of magnetic flux density (B) produced in a medium to the magnetizing force (H) producing it, than crystalline alloys. Also, as a class, the amorphous alloys are more magnetically "soft", i.e., exhibit lower coercive forces (Hc), than crystalline alloys. The unique magnetic properties of amorphous metallic alloys have led to proposals that magnetically soft amorphous metallic alloys be substituted for presently used magnetically soft crystalline metallic alloys in a variety of devices and applications. Candidates for the substitution include, for example, transformers of various sizes and types in which the cores are generally presently made of grain-oriented 3% silicon steels, magnetic delay lines, magnetic recording heads, and transducers such as stress and strain gauges.
Unfortunately, the highly desirable soft magnetic properties of amorphous metallic alloys are highly stress sensitive and deteriorate rapidly with increasing stress. This stress sensitivity is potentially limiting for many applications envisioned for amorphous metals and is particularly so for transformers and the like wherein the typically thin ribbon-like amorphous metal is wound on itself layer upon layer to form the core.
In general, stresses in amorphous metals may be classified as either residual or applied. Residual stresses typically result, at least in part, from the rapid cooling rates encountered during formation of the amorphous metal, as, for example, by the above-discussed chill-block melt spinning process. Due to their origin, those residual stresses are frequently referred to as "cast-in" or "quenched-in" stresses.
Applied stresses are those which result from the direct application of a load or are present due to the configuration to which the ribbon has been made to conform, e.g., ribbon wound on itself to form a transformer core. Both types of stresses are considered equivalent insofar as they are detrimental to the magnetic properties of amorphous alloys.
It is known that stresses in amorphous metals may be relieved by isothermal annealing. The maximum temperature allowable is that temperature at which the metastable glassy alloy begins to transform to its equilibrium crystalline state, i.e., the crystallization temperature (T.sub.x). The desirable soft magnetic properties of amorphous metals generally degrade rapidly with the onset of crystallization. Data in the literature indicates, however, that some crystallinity, i.e., on the order of about 2% or less, may be beneficial in some applications especially at frequencies greater than about several hundred Hertz.
The crystallization temperature is a function of several variables including, for example, local variations in composition. Unlike the melting point of a pure metal, the crystallization temperature of an amorphous alloy is generally not a fixed quality and may be influenced by the method used in its determination. Frequently, the crystallization temperature is determined by the method known as Differential Scanning Calorimetry (DSC) and then most frequently at a scanning or heating rate of 20.degree. C./min. Another technique commonly used with the DSC method is to determine T.sub.x at several scanning rates and report as T.sub.x the value determined by extrapolating to zero scanning rate. All too frequently, however, T.sub.x is reported without reference to its method of determination.
Therefore, due to the uncertainties in the determination of T.sub.x, it is preferable to anneal at temperatures sufficiently below T.sub.x to ensure that crystallization does not occur. As the stress relief annealing temperature decreases below T.sub.x, however, increases in annealing time, which are undesirable from a manufacturing productivity standpoint, are required. Excessive annealing times also promote brittleness.
The balance between isothermal annealing time and temperature is particularly critical and difficult to control in applications in which the ribbon has been formed to shape, e.g., wrapped to form a transformer core, prior to the anneal. When the core is subsequently annealed, each layer of the core experiences a different temperature-time history as the heat diffuses inwardly, thus yielding a non-uniform product. Further, once the ribbon has been annealed in a configured shape, it may not be reconfigured to a different shape without reintroducing detrimental stresses.
Various methods of stress relieving a moving ribbon have emerged in response to the difficulties experienced with batch isothermal annealing. One such method is taught by Senno et al. in U.S. Pat. No. 4,288,260, which is herein incorporated by reference. The method of Senno et al. principally comprises the continuous transfer of amorphous alloy ribbon between two stations over at least one heating body situated in-between the two stations such that at least one surface of the ribbon directly contacts the heating body during transfer.
Senno et al. set forth ranges for the rate at which the ribbon should be transferred over the heated body for which temperature ranges are also set forth. Generally, at a fixed transfer rate, the magnetic properties of the ribbon treated by the method of Senno et al. improve with increasing temperature of the heating body, but the improvement appears to be limited by the onset of crystallization. As the duration of the heat treatment increases, i.e., transfer rate decreases, the magnetic properties also generally improve. Thus, the magnetic properties of amorphous ribbons treated by the method of Senno et al. improve as the process conditions approach those of isothermal annealing. No critical interrelationship between transfer rate and the temperature of the heating body is shown. Senno et al. do not attribute any criticality to post-annealing operations, particularly ribbon handling operations, as the annealed ribbon is simply collected on a take-up reel.
Another method is taught by Satoh et al. in U.S. Pat. No. 4,284,441, which is also incorporated herein by reference. By the method of Satoh et al., internal stresses in a thin strip or ribbon of amorphous alloy are alleviated and the soft magnetic properties improved by alternately imparting tensile and compressive forces to the thin strip while the thin strip is maintained at a temperature within the range in which no deterioration of mechanical properties is induced. The alternate impartment of tensile and compressive forces to the thin strip of amorphous alloy is accomplished by causing the thin strip to be moved over at least one roller having a fixed radius of curvature. The treated ribbons are collected on take-up spools or bobbins and it is generally taught that the diameter of the bobbin should be greater than the diameter of any of the rollers used in the process. Satoh et al. provide only general teachings about such variables as roller diameter, the range of heating temperature and the travelling speed of the ribbon.
Satoh et al. further teach that the internal stress of the thin strip of amorphous alloy can be alleviated merely by moving the unheated thin strip at least once over a roller in such a manner that the ribbon surface opposite to the surface which came into contact with the chill surface is held in contact with the roller. Satoh et al. also teach that a magnetic iron core having good magnetic properties can be fabricated from ribbon treated by their method by rolling the treated ribbon into a core in such a way that the inside surface of the core is formed by the ribbon surface opposite to the surface which was in contact with the chill surface. That teaching is given without any regard to the variable stresses which might thereby be introduced as the radius of the wound object changes.